The efficiency of a solar cell may be its capability to convert the solar energy incident on it to electrical energy. The theoretical maximum efficiency of a single junction solar cell may be the ratio of the area of the rectangle with the longer side along the y axis representing the number of photons as a function of incident energy and the shorter side along the x-axis representing the energy to the area under the curve of energy vs. number of incident photons per unit area per second.
For silicon solar cell made of single crystalline silicon the theoretical maximum efficiency may be less than 30%. Remainder of the solar energy incident on a solar cell may be lost as heat.